As the number of electronic elements contained on semiconductor integrated circuits continues to increase, the problems of reducing and eliminating defects in the elements becomes more difficult. To achieve higher population capacities, circuit designs strive to reduce the size of the individual elements to maximize available die real estate. The reduced size, however, makes these elements increasingly susceptible to defects caused by material impurities during fabrication. These defects can be identified upon completion of the integrated circuit fabrication by testing procedures, either at the semiconductor chip level or after complete packaging. Scrapping or discarding defective circuits is economically undesirable, particularly if only a small number of elements are actually defective.
Relying on zero defects in the fabrication of integrated circuits is an unrealistic option, however. To reduce the amount of semiconductor scrap, therefore, redundant elements are provided on the circuit. If a primary element is determined to be defective, a redundant element can be substituted for the defective element. Substantial reductions in scrap can be achieved by using redundant elements.
One type of integrated circuit device which uses redundant elements is electronic memory. Typical memory circuits comprise millions of equivalent memory cells arranged in addressable rows and columns. By providing redundant elements, either as rows or columns, defective primary rows or columns can be replaced. Thus, using redundant elements reduces scrap without substantially increasing the cost of the memory circuit.
Because the individual primary elements of a memory are separately addressable, replacing a defective element typically comprises selecting a bank of switch circuits, each switch circuit typically being an antifuse or a fuse such that the bank is known as an antifuse bank or a fuse bank, respectively, to `program` a redundant element to respond to the address of the defective element, and then enabling the redundant element by programming an enable antifuse. This process is very effective for permanently replacing defective primary elements. A problem with this process, however, is the possibility of replacing a defective primary element with a defective redundant element. The possibility of having a defective redundant element increases as the number of redundant elements on an integrated circuit increases. Because the process of replacing defective elements is a permanent solution, if a defective redundant element is used, the circuit must be scrapped.
The number of redundant elements provided on a circuit usually exceeds the number of redundant elements needed to `repair` a defective chip. Therefore, it is desirable to replace the defective redundant element with another available redundant element. One manner by which to accomplish this is to include with the fuse or antifuse bank for each redundant element a cancel antifuse. If a redundant element proves to be defective, enabling the cancel antifuse effectively disables the fuse or antifuse bank, and therefore the redundant element. The fuse or antifuse bank for another redundant element can then be programmed to respond to the same address as the first redundant element to replace the defective primary element. However, this solution has a great drawback in that an additional antifuse is required for the fuse or antifuse bank of every redundant element, even though usually very few of the redundant elements are defective. Inclusion of a cancel antifuse for each redundant element is a poor use of die real estate.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for canceling and replacing defective redundant electronic elements on an integrated circuit without requiring a cancel antifuse for every redundant element.